(1) Field of the Invention
The present invention relates to an oscillator circuit in a semiconductor integrated circuit. The oscillator device according to the present invention is suitable for an oscillator circuit in which minimum distortion of the waveform of the output signal is required.
(2) Description of the Prior Art
In general, in an oscillator circuit formed in a semiconductor integrated circuit and having a crystal resonator connected externally, the peak-to-peak value of the input signal for the amplifier portion of the oscillator circuit is apt to exceed the voltage range "0-V.sub.CC " of the voltage source.
This phenomenon in the prior art oscillator circuit will be described with reference to FIGS. 1 and 2. On a P type semiconductor substrate 7, an amplifier circuit 1, composed of a depletion-type metal-oxide semiconductor (MOS) transistor 11 and an enhancement-type MOS transistor 12, and a feedback resistor 21 are provided. The feedback resistor 21 and a resonator element 22 of a crystal or ceramic type constitute a feedback circuit 2. Capacitors 31 and 32 are connected between the terminals of the resonator element 22 and ground. The output of the oscillator circuit is obtained from the connecting point 13 of the transistors 11 and 12 and is supplied through inverters 81 and 82 to a function block 83 as a timing pulse signal.
The operation characteristic of the oscillator circuit is illustrated in FIG. 2. The N-channel MOS transistors 11 and 12 are connected between V.sub.CC and V.sub.SS (=0).
Under ideal conditions of operation, the waveform of the input signal is shown as V.sub.i (1), which has a peak-to-peak value of V.sub.CC -V.sub.SS and a symmetrical shape with a duty ratio of 50% with respect to the level of the threshold voltage V.sub.T. The waveform of V.sub.i (1) is either rectangular or sinuosoidal. The waveform of the output signal is shown as V.sub.O (1), which has a peak-to-peak value of V.sub.CC -V.sub.SS and a rectangular shape with a duty ratio of 50%.
Under practical conditions of operation, the waveform of the input signal tends to become V.sub.i (2), which is shifted considerably to the upper side to give an asymmetrical shape. Hence, the waveform of the output signal tends to become V.sub.O (2), which does not have a duty ratio of 50%.
Although the output of the oscillator circuit of FIG. 1 is supplied through inverters 81 and 82 to a function block 83, such as a microprocessor circuit, such an output signal V.sub.O (2) having a duty ratio significantly less or greater than 50% is unsuitable because the function block 83 is prevented from properly operating.
Hence, the duty ratio of the waveform of the output signal of the oscillator circuit is required to be about 50%. Also, the duty ratio of the waveform of the input signal for the amplifier circuit portion of the oscillator circuit with respect to the input threshold voltage V.sub.T of the amplifier is required to be about 50%. Accordingly, prevention of asymmetry of the input signal for the amplifier portion with respect to the level of the threshold voltage V.sub.T is first required.
In the prior art oscillator circuit, a parasitic diode 5 formed by an N type diffusion region in a P type substrate for forming an input protection circuit (not shown) exists between input node 23 and the gate of the transistor 12, which is connected to the substrate at ground. When the gain of the feedback loop is sufficiently large to cause a large amplitude of the input signal at the input node 23 due to some uncontrollable parasitic reactive elements included in the resonator element 22, the parasitic diode 5 exerts a clamping effect to limit the fall of the waveform V.sub.i (2) at a portion lower than V.sub.SS within a predetermined value LIM(5) corresponding to the forward voltage of the diode 5, as illustrated in FIG. 2. However, the parasitic diode 5 does not exert a clamping effect enough to limit the rise of the waveform V.sub.i (2) at the upper portion within a predetermined value. Hence, the shape of the waveform V.sub.i (2) is asymmetrical, as shown in FIG. 2.
To counter the problem of asymmetry of the waveform V.sub.i (2), it has been contemplated to connect a clamping diode 6 to an input terminal 23 of the amplifier circuit 1. The clamping diode 6 would exert a clamping effect to limit the rise of the waveform V.sub.i (2) at the upper portion higher than V.sub.CC within a predetermined value LIM(6) corresponding to the forward voltage drop of the diode 6, as indicated by the broken line in FIG. 2. The clamping diode 6 would improve the situation because it would reduce the asymmetry in the waveform V.sub.i (2) with respect to V.sub.T.
However, the addition of a clamping diode 6 to the MOS type semiconductor integrated circuit is, in general, considerably difficult from the viewpoint of the manufacturing process and the MOS type integrated circuit structure. The addition of the clamping diode 6 to the input terminal 23 of the amplifier circuit 1 in an integrated circuit has not been realized so far. A discrete diode must be used for this purpose.